THE GRAIN BED COLLISION IN SAND TRANSPORT BY WIND

In the last decade much progress has been made toward the development of a comprehensive model of aeolian sediment transport in which the grain/bed collision has been identified as having a significant role. The grain/bed collision has been studied by both physical experiments and numerical simulation. A principal objective of these studies has been to gather sufficient data to characterize the collision in order that it may be represented as an empirical function in numerical models of the sand transport system. Thus the study of the ''act physical mechanism of the collision has to some extent been neglected. The transport of larger particles in saltation over a loose surface is known to promote the release of finer particles from that surface into suspension. Thus the precise physics of the grain/bed collision become highly significant with regard to the physics of dust release from a surface. This paper re-examines previous collision data and compares them with a simple collision model. This model proposed that the impinging grain strikes a single surface grain and rebounds: an alternative mechanism might be that the impinging grain ploughs through the surface striking a number of bed grains before rebounding. The collision data are shown to support the model: first, because the duration of the collisions observed on the high speed film supports a short contact time with the surface and, second, because the data fit well with the model.